In the first generation of recombinant xylose-utilizing S. cerevisiae strains the activity of the enzyme(s) converting xylose to xylulose has been insuffi­cient to support ethanolic fermentation of xylose [42,43,54] (strain RWB202, Tables 3 and 4). For example, overexpression of the nonoxidative PPP im­proved xylose fermentation only when the XR and XDH activities were enhanced [54,90] (strain TMB3057, Table 1, vs strain TMB3026, Table 3), indicating that when the flux through central metabolism was high, the control of xylose metabolism was in the steps converting xylose to xylu­lose [42] (strain TMB3050, Table 2). Invariably, increased XR and XDH ac­tivities have been observed in mutant S. cerevisiae strains with improved xylose utilization [31,57,91]. Similarly, high activity of Piromyces XI allowed higher xylose fermentation rates than the lower bacterial XI activity [42,92] (strain RWB202-AFX, Table 1; strain TMB3050, Table 2). Also, in recombi­nant arabinose-utilizing S. cerevisiae strains, enhanced levels of the arabinose isomerase significantly improved arabinose fermentation [8].

The fact that not only the cofactor specificities, but also the relative activ­ities of XR and XDH affect xylitol formation suggests that the redox model for xylitol formation [32] may have to be reevaluated. Not only the cofac­tor preferences of the enzymes involved but also, equally importantly, the levels of the XR and XDH activities affect xylitol formation during xylose fermentation [54]. A plain increase in XR and/or XDH activity, allowing an increased flux through the initial pathway, significantly reduces xylitol excretion [53,54,93] (strain TMB3062, Table 1; strain TMB3061, Table 2).

4.2.3